linux-kernel - [PATCH 4.4 106/162] workqueue: Fix pwq ref leak in rescuer

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Date:   Thu, 19 Dec 2019 19:33:34 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        "Williams, Gerald S" <gerald.s.williams@...el.com>,
        NeilBrown <neilb@...e.de>
Subject: [PATCH 4.4 106/162] workqueue: Fix pwq ref leak in rescuer_thread()

From: Tejun Heo <tj@...nel.org>

commit e66b39af00f426b3356b96433d620cb3367ba1ff upstream.

008847f66c3 ("workqueue: allow rescuer thread to do more work.") made
the rescuer worker requeue the pwq immediately if there may be more
work items which need rescuing instead of waiting for the next mayday
timer expiration.  Unfortunately, it doesn't check whether the pwq is
already on the mayday list and unconditionally gets the ref and moves
it onto the list.  This doesn't corrupt the list but creates an
additional reference to the pwq.  It got queued twice but will only be
removed once.

This leak later can trigger pwq refcnt warning on workqueue
destruction and prevent freeing of the workqueue.

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: "Williams, Gerald S" <gerald.s.williams@...el.com>
Cc: NeilBrown <neilb@...e.de>
Cc: stable@...r.kernel.org # v3.19+
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/workqueue.c |   13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2308,8 +2308,14 @@ repeat:
 			 */
 			if (need_to_create_worker(pool)) {
 				spin_lock(&wq_mayday_lock);
-				get_pwq(pwq);
-				list_move_tail(&pwq->mayday_node, &wq->maydays);
+				/*
+				 * Queue iff we aren't racing destruction
+				 * and somebody else hasn't queued it already.
+				 */
+				if (wq->rescuer && list_empty(&pwq->mayday_node)) {
+					get_pwq(pwq);
+					list_add_tail(&pwq->mayday_node, &wq->maydays);
+				}
 				spin_unlock(&wq_mayday_lock);
 			}
 		}
@@ -4276,7 +4282,8 @@ static void show_pwq(struct pool_workque
 	pr_info("  pwq %d:", pool->id);
 	pr_cont_pool_info(pool);
 
-	pr_cont(" active=%d/%d%s\n", pwq->nr_active, pwq->max_active,
+	pr_cont(" active=%d/%d refcnt=%d%s\n",
+		pwq->nr_active, pwq->max_active, pwq->refcnt,
 		!list_empty(&pwq->mayday_node) ? " MAYDAY" : "");
 
 	hash_for_each(pool->busy_hash, bkt, worker, hentry) {